Telephonic pulse-code-modulation system



y 1953 c. E. G. BAILEY TELEPHONIC PULSE-CODE-MODULATION SYSTEM FiledNov. 25, 1949 A INVENTOR. Edmund G'ervase Ende mirisl'opher AGENTPatented July 28, 1953 TELEPHONIC PULSE-CODE-MODULATION SYSTEMChristopher Edmund Gerva se Bailey, London, England, assignor toHartford National Bank and Trust Company, Hartford, Conn., as trusteeApplication November 25, 1949, Serial No. 129,226

In Great Britain November 26, 1948 9 illaims.

This invention relates to telephonic pulse-codemodulation systems andmore particularly to improvements in encoding devices therefore.

In such a system the instantaneous value of an audiofrequency phenomenon(hereinafter referred to, by way of example, as the speech voltage) atthe sending station is sampled at regularly recurrent intervals and isexpressed in terms of a. code combination of signal elements such asmarks and spaces. The combination is decoded at the receiving stationand is used for reconstructing an approximate reproduction of theoriginal speech voltage. If each code combination consists of n unitsthe number of diiierent combinations is 2 The value of the speechvoltage, on the other hand, varies continuously from 0 to, say, .80 -v.,so that it has in theory an infinite number of possible values and canbe only approximately represented by the values, finite in number,denoted by the code. Nevertheless it is found that, with quite a smallnumber of units per combination, a sufficiently undistorted reproductioncan be obtained. The system has the advantage that, since the decoderhas merely to determine presence or absence of a signal, the signal tonoise ratio can be made high and perfect signals can be regenerated byrepeaters.

Moreover, since a very brief period (say 1.5/ s. sufiiccs for eachsignal element, a large number of speech channels can be multiplexed ona timediyision or-distributor basis with a single carrier frequency. Anultra-high frequency carrier is preferably employed.

The code may be simply a binary system of numeration. Thus, in what maybe called the Straight code, if a marl; is denoted by 1 and a spaceby 0,the first unit 1 or 0 in a three-elementcombination stands for the value4 or 0, as the case may be, the second for 2 or 0 and the third for :1or 0.

With a three-element codeseven diiierent instantaneous values of thevoice-voltage can be signalled, in addition to zero. By way of examplethe present description will be restricted to'systems in which suchthree-element combinations are used, but it will be obvious that byincrease in the number of elements per combination any desired degree ofapproximation to a continuous series of voicevoltage can be obtained.

In the following description and claims the expression leinent periodwill denote the time allocated to one element (mark space) in a codecombination, and the expression combination period will denote the timeallocated to one complete combination of such elements. Thus in thesystems h eina r m re pa i ly described, by Way of example, acombination period will be three times as long as an element period.

The present invention relates more particularly to improvements in themeans for encoding the signals. Suitable transmission and multiplexingequipment andother components in a pulse.- code-modulation system Willbe known to those skilled in the art, and have been described in theBell System Technical Journal for July 1947 (page 395) and for January1948 (pages 1, 4e) and elsewhere.

In a known encoding device for impulse-codemodulation telephonycathode-ray tube provided with a coded stencil plate is employed. Such adevice is of value when combination signals containing relatively largenumbers of signal elements are used, so that the instantaneous values ofthe speech voltage can be finely graded into a relatively large numberof steps; for instance when the combinations cach comprise 7 elements sothat 128 steps are provided for. For many purposes, however, such asmilitary and police signalling, a much coarser.for instance an 8-step orlG-step-grading is adequate, and 3-element or Jr-element combinationswill accordingly give a sufficient degree of fidelity. In such cases aneconomy can be effected by discarding the cathode-ray tube and employinga circuit composed of ectifiers and other passive corn.- ponents, andone object of the invention is to provide an encoding device of thiskind.

One feature of the invention accordingly com prises an encoding devicefor a pulse-code-modulation system wherein the characters of all codeelements in each combination are determined by a control voltage whoseinitial value corresponds to the instantaneous value of the speechvoltage or other audioefrequency phenomenon to be signalled, while itssubsequent value decays during the combination period.

Briefly stated, a pulse-code-modulation system in accordance with apreferred embodiment {Of the invention is constituted by a samplingdevice and an encoding device.

The audio si nal to be coded is fed to the sampling device which isarranged to derive therefrom periodically a control pulse whose initialvalue is a function of the instantaneous value of the signal, thecontrol pulse thereafter decaying at a predetermined rate. Thus theresultant sawtooth shape of the control pulse is determined byinstantaneous value of the signal being sampled, the height of theleading edge of the pulse being a function of the instantaneous yalue 3of the signal, and the trailing edge having a fixed slope depending onsaid predetermined rate of decay.

The control pulse produced by the sampling device is then fed to theencoding device which contains a plurality of paralleled unidirectionalpaths, each of which includes two serially connected rectifiers. Thepaths have common input and output circuits, periodic code elementpulses bein supplied to the common input circuit at a rate determined bythe number of elements in the code combination. Thus if three elementsconstitute the code combination, for each sampling control pulse, threeelement pulses are applied to the input circuit.

The control pulse is imposed on the parallel paths and determineswhether or not any paths will be rendered conductive when an elementpulse is supplied thereto, and in that sense, the control pulsepreconditions said paths for conduction. The presence or absence ofconductive paths determines whether a mark or a space is developed inthe output circuit of the encoding device when an element pulse is fedtherethrough. In this fashion, each instantaneous value of the signal isconverted by the sampling device into a control pulse having a waveshape representative of said value, which control pulse, in turn, by itsform determines the code combination produced by the encoding device.

In order that the nature of the invention may be better understoodcertain embodiments thereof will now be described with reference to theaccompanying drawing, in which:

Figure 1 shows diagrammatically an encoding device in accordance withthe invention.

Figure 2 shows a circuit for modifying input Signals for use in thedevice shown in Figure 1.

The speech or modulation voltage is applied at the input terminals M,Figure 2, of a circuit, to be described later, which is controlled bysynchronising or channel-timing pulses so that it samples theinstantaneous value of the speech voltage once at the beginning of eachcombination period. Moreover, the circuit is such that its outputvoltage E, beginning at a level proportional to the value of the sample,falls off exponentially during the combination period. In the examplehere described, each combination is to comprise three code elements andthe rate at which E decays is arranged to be such that the voltage E ishalved during the time allocated to one element.

The output pulse voltage E yielded at the output terminals NW of thesampling circuit shown in Fig. 2 is applied asa control pulse to thesimilarly labelled input terminals MV of the encoding system shown inFig. 1, which encoding system yields a code combination depending on theform of the applied control pulse.

It is assumed, for the sake of example, that the voltage E ranges fromto 80 volts. Then its instantaneous values at the instant of samplingcan be classified in eight steps, numbered from 7 to 0 in column A inthe table, the first step comprising all those values which lie between80 and '70 volts, the second step all those between '70 and 60 volts,and so on, as shown in column B of the table. The value of E at thearrival of the first of the synchronizing pulses by which the emissionof the three code elements is timed (hereinafter called element pulses)will lie in one or other of the steps shown in column B. As a result ofthe exponential decay, however, the value of E will have fallen by onehalf by the time that the second element pulse arrives. The resultingvalues are shown in column C, so that any sample value which initiallylay between 80 and 60, for instance, will lie between 40 and 30 at theinstant when the second element is determined. At the arrival of thethird element pulse, E will again have been halved and will lie between20 and 15; it will therefore fall into the upper (20-10) of the twogroups shown in column D.

TABLE Encoding (Figure 2) Value of modulating voltage E at instant ofCode dements Code Step combilst ele- 2d ele- 3d elenation ment ment ment1st 2d 3d pulse pulse pulse 80-70 40-30 l l 111 70-60 20-10 0 l 011 A BO D E F G i H Now the circuit shown in Figure 1 is so 'arranged that if,at the instant when a synchronising or element pulse arrives atterminals EP, the value of E at terminals MV lies between 80 and 70, or60 and 50, or 40 and 30, or 20 and 10, a mark (herein denoted by l) issent on through the signal-element terminals SE; but if the volt age Elies between and 60, 50 and 40, 30 and 20, or 10 and 0, a space (denotedby 0) is sent on. As a result that first, second and third elements inthe combinations representing the grades shown in column A will be thosedenoted by 1 for a mark and 0 for a space in columns E, F and Grespectively, and the combination itself will be as shown in column H.These results are F earth. Three equal resistors R1, shunted bycapacitors C1, are connected between terminals P1 and P2 so thatpotentials above earth of 80, 60, 40

and 20 volts respectively are applied to the anodes of the four diodes(ii-d4. Similarly resistors Rs, shunted by capacitors C3, are soarranged that potentials above earth of '70, 50, 30 and 10 voltsrespectively are applied to the cathodes of the second set of diodes (isto (is. If now the instantaneous voltage E at terminals MV lies betweenand '70 the diodes (Z1 and (is will both be unblocked so that a verybrief impulse with a peak of one or twovolts, induced in the'twoparallel secondary coils of transformer T1 by a synchronising or elementpulse applied at terminals EP, can travel over the path P1 totransformer T2, where it induces a marking impulse which goes outthrough terminals SE to a trigger regenerator, and controls the emissionof a marking signal element.

On the other hand, when the voltage E lies between 70 and 60 noeffective path is available for a pulse from a transformer T1 totransformer T2. If, therefore, such a value of E obtains at the instantwhen an element pulse reaches terminals EP, no impulse passes and aspace is regisa. tered at the output tenninals SE. If the voltage E liesbetween 60 and 50 at the instant when an element pulse arrives, aneffective path 122 is open through diodes dz and do and a mark isregistered at terminals SE. Similar paths pa, n are respectively open atvoltages E lying between 40 and 30, and between 20 and 10, but no'efiective paths are open at the-remaining ranges of values.

Resistors R2 and capacitor C2 are provided for preventing theestablishment of an eiiective path for impulses through pairs of diodeswhich are not adjacent to one another. For instance a value 65 of thevoltage E opens a path through the diodes (Z1 and do, but this path isnot effectively available for the brief impulses of small amplitude bywhich the coding is effected, provided that'appropriate values be givento the components C2. R2. If it is found that the capacities of thediodes rovide a sufficient path to impair the functioning of the device,they can be balanced. out by means of a preset capacitor The manner inwhich the instantaneous speech voltage is sampled and in which therequisite decay rate is imparted to the resulting voltage E will now bedescribed with reference to Figure 2.

The modulation or speech voltage is applied at the input terminal M of acathode-follower valve Vi, so that a voltage proportional to theinstantaneous value of the speech voltage appears across the cathoderesistance Rs. Peaked positive synchronising pulses are applied to theterminal CP in such a way as to make the diodes D1, D2 momentarilyconductive at the beginning of each combination period. A resistor Raisconnected in series with the diode D2 and its resistance is chosen to bemuch smaller than that of the time-constant resistor shown at R4 butmuch larger than that of the cathode resistor R6. Since the resistanceof R5 is large in compariscn with the resistance of the remainder of thecircuit D2, D1, Re, most of the voltage of the controlling impulseapplied at CP is taken up in the voltage drop over it, R5, and thecapacitor C4 accordingly takes up the potential of the live end of thecathode resistor R6.

The pulses applied to terminal CP occur once at the beginning of eachcombination period, that is to say once for each three of the elementpulses applied at terminals EP, Figure 1. On the cessation of theimpulse at terminal C? the diodes D1, D2 become blocked and the initialcharge on the condenser G4 has thus been determined. This is the initialvalue of the control voltage E applied to terminals MV, Figure 1, whichare shown also in Figure 2, and it controls the character (mark orspace) of the first signal element in the combination in the manneralready described. The value 01 the voltage E across capacitor C4 thenfalls off exponentially, however, owing to the discharge of thecondenser through the resistance R4, and the latter is chosen to be ofsuch magnitude that the value of E is halved during a single elementperiod. The second and third signal elements are thus controlled by itin the manner already described, and hence the code shown in column H issignalled.

It will be obvious that by suitable dimensioning the resistors andcapacitors shown in Figure 1 the device can be adapted to effectvolumecompression for use in a compander system.

What I claim is:

1. In a pulse-code-modulator system, apparatus for encodin -audiofrequency signals comprising a sampling system to derive periodicallyfrom said audio signals a control voltage pulse whose initial value is afunction of the instantaneous value of the signals and whose valuethereafter decays at a given rate, and an encoding system coupled tosaid sampling system and responsive to each control voltage pulse togenerate a code combination having a predetermin d number of codeelements, the character of said elements being determined by the form ofsaid control voltage pulse.

2. Apparatus, as set forth in claim 1, which includes compander meansfor regulating the magnitude of said audio signals in a predeterminedfashion in order to produce audio signals with a predeterminedsignal-to-noise ratio.

3. Apparatus, as set forth in claim 1, wherein he control voltage pulsedecays at an exponential rate.

4. Apparatus, as set forth in claim 1, wherein the control voltage pulsedecays at a rate at which said voltage halves itself over the timeinterval in which a code element is generated.

5. Apparatus, as set forth in claim 1, wherein said sampling systemincludes a network for determining the rate of decay and having acapacitance and a resistance, said resistance shunt ing saidcapacitance.

6. In a pulse-code-modulator system, apparatus for encoding audiofrequency signals comprising a sampling system provided with a oathodefollower amplifier responsive to said audio signals and means to deriveperiodically from said amplifier a control voltage pulse whose initialvalue is a function of the instantaneous value of the signals and whosevalue thereafter decays at a given rate, an encoding system including aplurality of unidirectional current paths, means to supply each of saidcontrol pulses to said paths to precondition the conductivity of saidpaths in accordance with the control pulse values, and means to applyperiodic timing pulses to said paths in the interval during which saidpaths are conditioned by a control pulse for rendering said pathsconductive and non conductive in a sequence depending on said controlpulse values thereby producing a code combination having a predeterminednumber of elements.

7. In a pulse-code-modulator system, apparatus for encoding audiofrequency signals comprising a sampling system provide with a cathodefollower amplifier responsive to said audio signals, and means to deriveperiodically from said amplifier a control voltage pulse whose initialvalue is a function of the instantaneous value of the signals and whosevalue thereafter decays at a given rate, an encoding system including aplurality of parallel unidirectional current paths intercoupling thecommon input and output circuits, each of said paths including at leasttwo serially-connected rectifiers, to supply each of said control pulsesto said parallel paths to condition the conductivity of said paths inaccordance with the control pulse values, and means to apply periodictiming pulses to said common input circuit in the interval during whichsaid paths are preconditioned by a control pulse for rendering saidpaths conductive and non-conductive in a sequence depending on saidcontrol pulse values thereby yielding in the output circuit a codecombination having a predetermined number of elements.

8. Apparatus, as set forth in claim 7, wherein the ends of eachrectifier adjacent to the common input and output circuits of" saidparallel paths are coupled to points of constant potential, said pointsof constant potential having graded magnitudes. I

9. Apparatus, as set forth in claim 8, wherein said input and outputcircuits are constituted by transformers, the rectifier ends beingcomiected to said points of constant potential through a winding of arespective transformer.

CHRISTOPHER EDMUND GERVASE BAILEY.

References Cited in the file of this patent- UNITED STATES PATENTSNumber Name Date Haynes Sept. 5, 1944 Goodall Sept. 14, 1948 Lesti Sept.12, 1950 Cole Feb. 13, 1951 Lesti June 12, 1951

